The IC tag is considered to be a product positioned at the entrance to the ubiquitous age. Name tags, Suica cards, FeRAM cards, and the like have been developed for RF-ID (microminiaturized radio frequency identification). Many people expect that the IC tag market must grow greatly. However, the IC tag market has not yet grown more than expected. This is because there are social problems that should be overcome, such as cost, security, and confidentiality.
It is also expected that the RF-ID technology is applied to the identification of written property, such as paper money and securities. Since the problem of counterfeit bills becomes significant, an IC tag may be embedded in paper money to solve such a problem. However, IC tags are expensive and large. It is therefore difficult to embed an IC tag.
The price of IC tags can be reduced by miniaturizing IC tag chips. The miniaturization of IC tag chips results in an increased number of tag chips produced from a single wafer. So far a 0.4 mm square IC tag chip has been developed. This IC tag chip can store 128-bit memory data that can be read by microwaves of 2.45 GHz (see, for example, Non-patent Document 1).
Radio frequency automatic identification (RF/AID) systems have also been developed which use elements other than the IC tag and can be applied to identification of paper money and credit cards. Patent Document 1, for example, uses a plurality of resonators resonating at a plurality of radio frequencies, fixed in a random fashion onto a paper or plastic substrate. The plurality of resonators are of passive solid. If quartz is used, the resonance frequency is varied depending on the crystal orientation and the size of the quartz crystal. Hence, resonators having different shapes resonate at different frequencies. The above passive solid resonator includes a thin dipole made of an elongated metal. More specifically, the passive solid resonator is made of a quartz family, such as quartz crystal. For a radio frequency target, a plurality of resonators disposed on a substrate resonate when they are irradiated with electromagnetic waves having a radio frequency. The positions of the plurality of resonators can be known by detecting this resonance. The target is thus identified.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 10-171951    Non-patent Document 1: Mitsuo USAMI “An ultrasmall RFID chip: μ-chip”, OYO BUTSURI, Vol. 73, No. 9, 2004, pp. 1179-1183